1. Field of the Invention
The present invention relates to a thin film transistor device including a thin film transistor driven by a low voltage and a thin film transistor driven by a high voltage, and to a method of manufacturing the same. More specifically, the present invention relates to a thin film transistor device applicable to a liquid crystal display panel, which includes a peripheral circuit provided with thin film transistors and disposed outside a display region.
2. Description of the Prior Art
Liquid crystal display devices have lightweight, thin profile, and low power consumption characteristics and are therefore used in various electronic devices such as display devices for personal digital assistants (PDAs), notebook personal computers, desktop personal computers and the like, or viewfinders for video cameras. Particularly, an active matrix liquid crystal display devices including thin film transistors (TFTs) provided in respective picture elements (subpixels) as switching elements exhibits a high drive capability and an excellent display characteristic.
In general, a liquid crystal display device includes two substrates and liquid crystal which is sealed between these substrates. Picture element electrodes, TFTs, and the like are formed for respective picture elements (subpixels) on one of the substrates, while color filters opposed to the picture element electrodes and a common electrode common to the respective picture elements are formed on the other substrate. The substrate including the picture element electrodes and the TFTs formed thereon will be hereinafter referred to as a TFT substrate, and the substrate to be disposed opposite to the TFT substrate will be hereinafter referred to as a counter substrate. Moreover, a structure formed by sealing the liquid crystal between the TFT substrate and the counter substrate will be hereinafter referred to as a liquid crystal display panel.
In recent years, a peripheral circuit integrated liquid crystal display panel including peripheral circuits such as drivers (drive circuits) formed outside a display region is drawing attention. In the peripheral circuit integrated liquid crystal panel, it is necessary to form a semiconductor film constituting an active layer of the TFTs with polycrystalline silicon in order to form the TFTs having high drive capabilities.
When forming the polycrystalline silicon TFTs, a high density of an impurity is doped in a contact region by use of an ion doping device or the like, and then an activation process is carried out. The activation process includes a laser activation process of irradiating a laser beam which is outputted from a pulse oscillation excimer laser, and a thermal activation process of activating the impurity by a thermal process. In the laser activation process, the laser beam is irradiated beyond gate electrodes. Accordingly, metal having high reflectivity is used as the material for the gate electrodes. Meanwhile, in the thermal activation process, the contact region is heated at a high temperature equal to or above 500° C. Accordingly, metal having a high melting point is used as the material for the gate electrodes.
Incidentally, a display device used in a portable terminal or the like is required to be operable with low power consumption. Accordingly, the peripheral circuits are expected to be operable with low voltages. For this reason, it is preferable to form the peripheral circuit with TFTs having thin gate insulating films. However, in terms of the liquid crystal display panel, a voltage in a range from 7 to 10 V or above (a display voltage) is applied to the picture element electrodes through the TFTs to ensure the voltage required to drive the liquid crystal. Accordingly, it is necessary to increase the thickness of gate insulating films of the TFTs in a range from 80 to 150 nm to ensure the gate voltage resistance. For this reason, the peripheral circuit integrated liquid crystal display panel has a problem in that the peripheral circuits are formed with the TFTs requiring high operating voltages and power consumption is therefore increased.
To solve this problem, the inventors of the present invention have proposed to change the thickness between the gate insulating films of the TFTs at a display portion (hereinafter referred to as picture element TFTs) and the gate insulating film of the TFTs in a peripheral circuit portion as disclosed in Japanese Unexamined Patent Publication No. 2003-188183, for example. The TFT having the thinner gate insulating film will be hereinafter referred to as a low voltage drive TFT, and the TFT having the thicker gate insulating film will be hereinafter referred to as a high voltage drive TFT.
FIG. 1A is a cross-sectional view showing the low voltage drive TFT formed in the peripheral circuit portion of the liquid crystal display panel, and FIG. 1B is a cross-sectional view showing the high voltage drive TFT formed at the display portion of the liquid crystal display panel. A method of manufacturing the conventional thin film transistor device (the liquid crystal display panel) will be described with reference to FIGS. 1A and 1B.
Firstly, a glass substrate 11 is prepared as a base for a TFT substrate, and a silicon nitride (SiN) film 12 and a silicon oxide (SiO2) film 13 are formed on this glass substrate 11 collectively as a base film.
Next, an amorphous silicon film is formed on the SiO2 film 13 of the base film. Then, a laser beam is irradiated on the amorphous silicon film to form a polycrystalline silicon film 14. Thereafter, the polycrystalline silicon film 14 is patterned into a predetermined shape by use of the photolithography method.
Next, a SiO2 film 15 is formed on the entire upper surface of the glass substrate 11, and then the SiO2 film 15 located in a low voltage drive TFT forming region is removed by patterning. Thereafter, a SiO2 film 16 is formed on the entire upper surface of the glass substrate 11, and a conductive film made of an Al alloy such as aluminum neodymium (Al—Nd) is further formed thereon.
Next, a resist film is formed into a predetermined shape on the conductive film, and the conductive film and the SiO2 films 15 and 16 are etched by using this resist film as a mask. In this way, gate electrodes 17a and 17b of a low voltage drive TFT and a high voltage drive TFT, and a gate bus line are formed. Here, the SiO2 film 16 constitutes a gate insulating film in the low voltage drive TFT forming region, and the laminated SiO2 films 15 and 16 constitute a gate insulating film in a high voltage drive TFT forming region.
Thereafter, an impurity is ion-implanted into the polycrystalline silicon film 14 for forming a high density impurity region 14a constituting a source and a drain of the low voltage drive TFT, and a high density impurity region 14b constituting a source and a drain of the high voltage drive TFT. In this case, in the high voltage drive TFT forming region, it is also possible to form a low density impurity region (a lightly doped drain, or LDD) 14c between the high density impurity region 14b and a channel region by utilizing a difference in levels between the gate electrode 17b and the gate insulating film (the SiO2 films 15 and 16) as shown in FIG. 1B.
Next, a laser beam is irradiated onto the polycrystalline silicon film 14 doped with the impurity to activate the impurity. Thereafter, a SiO2 film 18 is formed on the entire upper surface of the glass substrate 11. Then, after forming contact holes on the SiO2 film 18, a metallic film made of Al or an Al alloy is formed on the entire surface thereof. Source and drain electrodes 19a and 19b and a data bus line are formed by patterning this metallic film. Subsequently, a SiN film 20 is formed on the entire upper surface of the glass substrate 11 to cover the TFTs. Moreover, an insulative organic resin film 21 is formed thereon. In this way, it is possible to form the peripheral circuit integrated liquid crystal display panel including two types of TFTs having different thickness in the gate insulating films.
However, in the above-described method of manufacturing the conventional thin film transistor device, the gate electrodes of the TFTs and the gate bus line are made of Al or the Al alloy. Accordingly, although a resistance value of the gate bus line is low, it is not possible to activate the impurity by the thermal activation process. The impurity has to be activated by the laser activation process. Nevertheless, it has been proved that activation of the impurity by the thermal activation process is less susceptible to an influence of hot carrier deterioration as compared to activation of the impurity by the laser activation process, and that the thermal activation process is more effective to form the reliable TFTs.
It is also conceivable to form the gate electrodes with metal having a high melting point so as to enable the thermal activation process. However, the high melting point metal causes an increase in resistance of the gate bus line particularly in the case of a large-sized liquid crystal display device. Such an increase in resistance may cause damping of signals which may result in incapability to drive the TFTs. It is also conceivable to increase the width or the thickness of the gate bus line in order to reduce the resistance. However, in that case, there arises a new problem of a difficulty to achieve high fineness.